There are several rules for attacking that determine if you can attack a player.

1) if you or your target are under 50,000 player score (PS), then the 5x's rule applies to you. The 5x's rule is that you can only attack a player if they are between 1/5 your PS and 5x's your PS.

Eg. You have a score of 20,000 PS. Your lower limit score for attacking and being attacked by is 4,000. Your upper limit for attacking or being attacked by is 100,000 PS.

2) if your score is greater than 50,000 PS, you have no upper limit score you can attack or be attacked by, however, the lower limit still follows the 5x's rule.

Eg. Your score is 50,000 PS. A 999,999,999 PS player can attack you and you can attack them. Your lower limit for attacking or being attacked by is 10,000 PS.

Eg. If your score is 250,000 PS or higher, you cannot attack or be attacked by a player lower than 50,000 PS. There is no upper limit PS that you can attack or be attacked by.

So in regards to your question as to why you cannot attack the player, I would assume that 1) that players score is below 50,000 PS and 2) that players score is also lower than 1/5 your PS. This means that players score is too low for you to attack.